Skateboard Wheels

1. Skateboard Wheels Case Study CHE 620

2. Introduction • From the first push forward on a home driveway to intricate freestyle routines in a purpose-built park, and at all levels in between, skateboarding is a very exhilarating and fun-filled activity. • In synthesizes the best features of many sports, including surfing, skiing, dance, gymnastics, and martial arts, combining strength, grace, and concentration.

3. Main Parts of a Skateboard • Three main parts • the deck or flexboard • the wheels • the truck

4. Main Parts of a Skateboard • Common skateboard decks • (a) basic flat board • (b) kicktail board for rear foot leverage • (c) rocker board for lower center of gravity • (d) cambered board for slalom and downhill racing

5. Main Parts of a Skateboard • Common deck materials • wood (ash, teak, oak) • polymers (polypropylene, polycarbonate) • composites (glass fiber reinforced epoxy resin) • wood and fiberglass combinations • aluminum and its alloys

6. The Wheels • Modern skateboarding probably would not be possible without major advances in both the design of the wheels and the materials used in their manufacture. • The wheels generally range in size from 29 mm width by 38 mm diameter to 58 mm width by 67 mm diameter.

7. The Wheels • The wheels contain either loose ball bearings or double-sealed precision bearings. • The former have the disadvantage of requiring regular lubrication and the possibility of the wheels being slowed by sand, road grit, and other particles.

8. The Wheels • Connecting the deck to the wheels is a steering and suspension system called a “truck.” • While trucks come in different designs, they all feature “shock absorbers” of elastomeric or polymeric bushings or cushions or metallic coiled springs. • Some trucks are adjustable in length and can be tightened or loosened to accommodate the weight of the rider.

9. The Wheels • Various parts of the truck are made of cast iron, forged iron, heat-treated aluminum, or steel. • (a) standard wheel with open loose bearings • (b) flex wheel with double-sealed precision bearings

10. Materials Selection Considerations • To perform satisfactorily, the materials for the wheel (and bearing assembly) must have the following characteristics • lowest possible rolling friction resistance, in order to maximize the speed of the skateboard (the primary requirement) • high frictional resistance to lateral movement (important for slalom and freestyle motions) • good shock absorption qualities to make riding on rough surfaces more comfortable • high resistance to abrasive wear • high resistance to corrosives such as lubricating oil

11. Materials Selection Considerations • At right is shown the contact stress on a perfectly elastic wheel in motion on a rigid surface and carrying a static load. • Upon rolling, one part of the “flat” suffers an increase in strain while the strain on other parts is reduced.

12. Materials Selection Considerations • For “real” materials, however, the stress in the section of the flat where the strain is constantly decreasing lags behind that in the other section of the flat, resulting in a stress distribution as shown at right.

13. Materials Selection Considerations • The final outcome is that the center of pressure, O¢, moves forward of the center of the wheel O by a small amount s, giving rise to a bending moment, Ps, that resists rolling.

14. Materials Selection Considerations. • The rolling resistance force R is the equivalent force acting at the center of the wheel in the direction opposite to the bending moment: where Dw is the diameter of the wheel

15. Materials Selection Considerations • The coefficient of friction, mrs is thus

16. Materials Selection Considerations • Some knowledge of s is needed in order to estimate mrs from the above equation. • For a perfectly elastic wheel material, s = 0. • For a “worst case” material, with the stress distribution shown at right, s = ac, where c is the half-width of the flat and a is a numerical constant.

17. Materials Selection Considerations • For many “real” materials, therefore, we can assume that, as a first approximation, s = a(1-A)c where A is the percentage rebound and is equal to the square of the coefficient of restitution e of the wheel material.

18. Materials Selection Considerations • Now where b is a numerical constant, usually about 0.8, h is the width of the wheel, and Eef is the effective modulus of the wheel.

19. Materials Selection Considerations • Eef is given by where Ew and nw are the modulus of elasticity and Poisson’s ratio of the wheel material, respectively, and Ers and nrs are the corresponding values for the road surface material.

20. Materials Selection Considerations • From the above, we can obtain an expression for mrs

21. Materials Selection Considerations an examination of the above shows that, for a given wheel with specified values of wheel area (h x Dw) and wheel load P, the primary design requirement of low friction may be obtained by using a material with high values of modulus of elasticity and coefficient of restitution

22. Materials Selection Considerations • On rougher surfaces, such as tarmac or concrete, an important additional factor affects friction, which leads to a reconsideration of the desirable properties for a wheel material. • Upon rolling over a rough surface, the wheel load will be supported on road asperities where the contact pressure is high. • The resulting penetration must be countered by the elastic deflection of the wheel if energy loss and friction are to be minimized. • This requires a wheel material with a large elastic strain and high elastic modulus, with the former becoming more important as road roughness increases. • For a rough surface, mrs is increased by about 50% (mrs’ = 1.5mrs)

23. From the above, the best material from the viewpoint of both mrs and mrs’ is glass fiber reinforced polyimide. • However, because of its relatively high cost, low shock absorption qualities, and relatively low wear resistance, the material currently favored for the construction of skateboard wheels are hot-cured polyether-based polyurethanes.

24. The End • The End